In general, inkjet printing machines or printers include at least one printhead that ejects drops or jets of liquid ink onto a recording or image forming surface. An aqueous inkjet printer employs water-based or solvent-based inks in which pigments or other colorants are suspended or in solution. Once the aqueous ink is ejected onto an image receiving surface by a printhead, the water or solvent is evaporated to stabilize the ink image on the image receiving surface. When aqueous ink is ejected directly onto media, the aqueous ink tends to soak into the media when it is porous, such as paper, and change the physical properties of the media. To address this issue, indirect printers have been developed that eject ink onto a blanket mounted to a drum or endless belt. The ink is dried on the blanket and then transferred to media. Such a printer avoids the changes in media properties that occur in response to media contact with the water or solvents in aqueous ink. Indirect printers also reduce the effect of variations in other media properties that arise from the use of widely disparate types of paper and films used to hold the final ink images.
In these indirect printers, the blanket surface must wet well enough to prevent significant coalescence of the ink on the surface and also facilitate the release of the ink from the blanket to the media after the ink has dried on the blanket. Applying a coating material to the blanket can facilitate the wetting of the blanket surface and the release of the ink image from the blanket surface. Coating materials have a variety of purposes that include wetting the blanket surface, inducing solids to precipitate out of the liquid ink, providing a solid matrix for the colorant in the ink, and/or aiding in the release of the printed image from the blanket surface. Because the blanket surfaces are likely to be surfaces with low surface energy, reliable coating is a challenge. If the coating is too thin, it may fail to form a layer adequate to support an ink image. If the coating is too thick, a disproportionate amount of the coating may be transferred to the media with the final image. Image defects arising from either phenomenon may significantly degrade final image quality.
In previously known indirect printers, operators observe the ink images on the media output by the printer and evaluate the quality of the ink images. The operator can adjust various parameters for the printer and repeat the evaluation of the image quality. Once the operator determines the image quality is adequate, the operator commences a print run. Such trial-and-error techniques are prone to operator subjectivity and color sensitivity. Improvements in aqueous indirect inkjet printers that enable more objective evaluations and consistent coating layers are desirable.